Anionic disulfonate surfactants may be prepared by Strecker sulfonation of alkyl di-chlorides with hydrophobic tail lengths of eight to sixteen carbons. Such surfactants contain excess inorganic salts as reaction by-products. The reaction may be generally described by equation (1) below:RCl2+2M2SO3→R(SO3)2M+MCl  (1)where M is a metal such as Na, K, and R is an alkyl group having between eight and sixteen carbons.
Strecker sulfonation may also be used to sulfonate halogenated alkyl ethers. Halogenated alkyl ethers may be obtained from the acid catalyzed etherification of halogenated alkyl alcohols with α-olefins. For examples, mono- and di-sulfonate surfactants may be produced from Strecker sulfonation of alkyl ethers of 1,3-dichloro-2-propanol (DCP), as shown in equation (2):
The reaction shown in equation (2) may be catalyzed by one or more metal halides, including for example sodium iodide. While equation (2) illustrates the DCP ether as being chlorinated, such Strecker sulfonation processes may be carried out with ethers having other halogen substituents, including for example, fluorine, iodine and bromine.
Inorganic salts are a by-product of such Strecker sulfonation processes. In fact, inorganic salts, typically sodium chloride, sodium sulfite and sodium sulfate, may constitute between 50 and 60% by weight of the sulfonation process product on a dry basis. Other reaction products of the Strecker process shown in equation (2) include non-polar organic species such as the unreacted halogenated alkyl ethers, alkyl alcohols from competitive hydrolysis, and long chain alkenes from dimerization of the α-olefin during etherification.
Such high levels of inorganic salts in surfactant solutions, however, may limit the use of the disulfonate surfactants in some applications such as emulsion polymerization, cleaning formulations, or personal care product formulations because inorganic salts may affect surfactant properties. Nevertheless, such disulfonate surfactants display other properties, such as biodegradability, hydrolytic and formulation stability, which are highly desirable.
Inorganic salts have been removed from anionic surfactant solutions by electrodialysis using a hydrophilic neutral membrane and a cation exchange membrane. Such removal process requires specialized processing equipment which significantly raises the cost of producing surfactants for applications where high inorganic salt levels cannot be tolerated. An additional drawback to using membrane technology for inorganic salt removal from anionic surfactant solutions is the generation of a large volume waste stream, typically four to six times the initial volume of the surfactant solution to be treated, containing low levels of surfactant product, along with the associated product loss. Consequently, a commercially facile and efficient method to remove or reduce the amount of residual inorganic salts in disulfonate alkyl surfactant solutions, particularly those arising from the sulfonation of halogenated alkyl ethers would be desirable. In addition to lowering the salt content of disulfonate surfactants, it is often desirable to produce aqueous concentrates of surfactants, especially because Strecker sulfonation process produces surfactant solutions having a relatively low surfactant concentration, i.e., ranging from 5 wt % to 20 wt % surfactant. Therefore, an economical and commercially practical process for concentrating the surfactant in disulfonate surfactant solutions would also be highly desirable.